Considering that the maximum possible machine width will soon be reached in today's modern paper machines, the operating speed thereof has to be further increased if the productivity is to be increased. However, since higher machine speeds will result in higher web stresses and, consequently, in an increased risk for web breakage, the machine speed must be limited to such an extent that the maximum tensioning of the web is not exceeded. The situation becomes even more complicated if one also considers the more frequent use of recycled fibres, which have lower strength than virgin fibres.
There are numerous examples in the prior art on how to obtain higher machine speeds by eliminating so called open draws from the paper machine. The term "draw" refers to the manner in which the web is transferred from one drive section to another in a paper machine. A draw is called a "closed draw" if the web is supported at least on one side at the point of transfer, e.g. by a felt. In other cases, it is called an "open draw". The supporting element, traditionally in the form of a felt, a woven fabric or a support drum, is especially useful for transferring a wet or moist web. In some positions of the paper machine there is a requirement for the smoothness of the drum as well as for the supporting length of the felt; in such cases belts having a coated web-contacting surface are used.
It has also been proposed to introduce some form of web supporting elements at previously unsupported positions of the paper machine, such as in a coating station.
In a conventional coating station, an amount of coating material is applied to a moving web. Known coating station may be arranged to apply the coating material on one or both sides of the web. The coating station may be located either "on-line" in a paper machine, normally just before a calendering station or a reeling-up station of the paper machine, or "off-line" separate from the paper machine. In the latter case, the web speed in the coating station must preferably be higher than the web speed in the paper machine. Otherwise, the coating station may become a "bottleneck" in the overall production line.
U.S. Pat. No. 4,761,309 (Beloit) discloses the use of a fabric as a backing element in a short dwell coating station. The object is to avoid so called air blistering, that is the formation of air pockets which are developed between a conventional backing roll and the web upstream relative to the short dwell coater when the web is moving at high speeds (&gt;3000 feet/minute) and air is sucked into the coater unit.
The arrangement in U.S. Pat. No. 4,761,309 includes a backing roll having a perforate surface and a permeable belt disposed between the perforated roll surface and the web. When a vacuum is applied, the web is drawn into close contact with the belt thereby avoiding the formation of said air pockets between the web and the backing member. The permeable belt is a wire-mesh belt including two layers of different denier, of which the finer layer is in contact with the web in order to avoid marking.
WO90/11135 (Beloit), mentioned in the first paragraph, discloses the use of a "backing blanket" in a coating station. The backing blanket is guided in an endless path around two guide rollers which are spaced apart, so that the web, which is supported by the blanket, is running in a plane oriented tangentially relative to the guide rollers. A short dwell coater is disposed between the guide rollers and adjacent the web, on the side thereof facing away from the rollers, for applying a coating material on the web.
The backing blanket is of composite construction, including a woven base fabric and a surface layer in the form of a woven material, which is positioned in a plane parallel to the base fabric and has a lower denier (finer) than the denier of the base fabric (coarser).
The object of the arrangement disclosed in WO90/11135 is to accomplish a more even distribution of the coating material, by avoiding the occurrence of secondary flow of the coating material in opposition to the primary flow. By the use of said backing blanket, centrifugal forces on the coating material generated by conventional backing rollers are said to be avoided.
WO95/14816 (Valmet) discloses a tail threading arrangement in a coating station. The tail threading process, which is performed during start up and after web breaks, involves the step of providing an edge strip slit from the web to act as a "tail" of the web which is first threaded through the line and then widened to the normal width of the web. The object of the arrangement in this document is to positively support the web through essentially the entire machine and to have only very narrow open draws. To this end, the disclosed arrangement comprises a support belt in a coating station. The web to be coated enters onto said support belt from a delivering wire, so that the web travels together with the support belt through the coating station. The exposed side of the web not facing the support belt is coated with a coating material.
According to the teachings of this prior-art document, a support belt used in coating station must have a very smooth surface to keep the paper coating profile level. It is stated that the support belt material should have a smooth surface or maximally containing small-diameter micropores. The term belt is defined as any non-air-permeable, flat support element. The document gives no further information on the construction or materials of the support belt.
To the knowledge of the inventors of the present invention, the arrangement described in WO95/14816 has not been used in practice.
It is stated in WO95/14816 that since web adherence cannot be arranged by vacuum, additional support can be provided by air-jets. It is also stated that the web will tend to adhere relatively strongly to the smooth surface of the support belt, initially by static electricity, and after the coating application, adhesion is caused by the moisture of the web.
EP-A-0 576 115 (Albany International Corp.) discloses a transfer belt for carrying a paper sheet from a press nip in a paper machine to a transfer point.
Although this prior-art transfer belt in some aspects operates according to the same principles as the invention, there are in fact substantial differences. The operation of the transfer belt according to EP-A-0 576 115 requires a compression of the belt in order to accomplish a web-release function at the transfer point. More specifically, the transfer belt comprises a system of polymers and hard particles embedded therein. When the belt is compressed in the press nip, it becomes very smooth giving a good web contact. On the exit side of the nip the system expands, but differently at soft and hard regions. Thereby, the water film between the belt and the web is split or broken and the web can be released. This technique based on a pressure responsive belt cannot be used in a coating station, because there is not any substantial compressive pressure acting on the web-belt system in a coating station. Furthermore, the moisture content of a web in a press section being essentially higher (the moisture content appoximately 80% on the entrance side and 50% on the exit side) than that in a coating station (moisture content approximately 5-10% on the entrance side), the operating environment of this prior-art transfer belt and the invention are essentially different. The environment is nearly 100% dry in a coating station and, therefore, web handling problems in a coating station are caused by different factors than in the "wet" environment in a press section.